The present invention relates generally to an integrated fuel cell power plant, and more specifically to a combination of cycles wherein a first fuel cell cycle tops an indirect-fired gas turbine cycle and a second fuel cell cycle bottoms the gas turbine cycle so that the cycles are thermally integrated in a tandem operating arrangement.
Gas turbines and fuel cells are well known mechanisms used for the production of electric power. Gas turbine cycles and fuel-cell cycles have each been previously bottomed with steam turbine cycles for the co-generation of electric power. With cycles employing a fuel cell, to pressurize the system, compressors have been used to provide a stream of pressurized air to the cathode of the fuel cell. Full advantage was not taken of the pressurized air until assignee's U.S. Pat. No. 5,449,568 referenced hereinbelow, wherein the air stream is heated with the exhaust stream from a molten carbonate fuel cell bottoming a gas turbine. To enhance fuel cell performance, carbon dioxide is added to the air stream with the resulting mixture undergoing an electrochemical reaction with fuel introduced at the anode of the fuel cell. The fuel cell produces electrical energy and provides streams of hot gases used for generating steam for a bottoming steam turbine cycle that is coupled to a suitable electric generator.
A variation of such a combined cycle includes a fuel cell bottomed with a gas turbine instead of the steam turbine cycle so that residual heat energy in the cathode exhaust stream can be directly extracted in the gas turbine for the production of electric power. Such a system is described in assignee's U.S. Pat. No. 4,921,765 to Gmeindl et al which issued May 1, 1990 and is incorporated herein by reference thereto.
In another variation of gas turbine and fuel cell combined cycles, a direct-fired gas turbine cycle is combined with a fuel cell cycle for producing the hot gas stream used for driving the gas turbine and thereby providing generation of electric power in both the gas turbine cycle and the fuel cell cycle.
While these previously known combined cycles do provide for the co-generation of electric power, these systems have not been found to be capable of providing power conversion at high efficiencies. The primary reason for this short-coming is that presently available fuel cells, such as molten carbonate fuel cells as described in assignees aforementioned patent, can only operate at pressures up to about six atmospheres so as to provide turbine-driving gas streams at pressures less than required for efficient operation of a gas turbine.
In an effort to improve the efficiency of these types of combined cycles, an indirect-fired gas-turbine cycle was bottomed with a molten carbonate fuel-cell (MCFC) cycle wherein the gas turbine is operated at optimum pressure for efficient power conversion. In this combined cycle arrangement, the gas turbine is driven with indirectly heated air at a pressure appropriate for efficient operation of the turbine, while the turbine exhaust, which is at a pressure considerably less than that at the gas turbine inlet, is directly utilized in the fuel cell cycle for the electrochemical reaction. Such a system is described in assignees U.S. Pat. No. 5,449,568 issued Sep. 12, 1995 to Micheli et al and is incorporated herein by reference thereto.
The system of the above referenced patent application includes a compressor for providing a stream of compressed, preheated air to the gas turbine. The compressed air is heated in an indirect heat exchanger which is supplied with a stream of heated gases from a combustor to which auxiliary fuel is supplied along with the exhausts from the molten carbonate fuel cell electrode chambers. The cathode chamber inlet of the fuel cell is connected to the gas turbine exhaust to receive the stream of heated air discharged therefrom. The main fuel feed is supplied to the anode chamber inlet of the fuel cell in the form of a gaseous hydrocarbon fuel, such as fuel gas or natural gas. This fuel is internally reformed into hydrogen and CO at the cell anode for effecting the electrochemical reaction with the stream of heated air supplied to the cathode thereof for the galvanic production of electrical energy. The heated gases at the anode and cathode exhausts are used to provide at least a portion of the heat for the incoming pressurized air stream in the heat exchanger.
To produce sufficient CO.sub.2 for the operation of the MCFC, a portion of the hot gas stream from the cathode is combined in a suitable catalytic reactor with the stream of hot gases, including residual fuel values from the anode, for the production of carbon dioxide. The carbon dioxide in the discharge stream of hot gases from the catalytic reactor is separated in a CO.sub.2 separator and mixed with the hot air stream discharged from the gas turbine and fed to the cathode reaction chamber of the fuel cell.
The heat value from the stream of the heated gases that are discharged from the heat exchanger can be further utilized for power generation in various ways to further improve the system efficiency. Also, by using the hot exhaust gases from the fuel cell for partially heating the compressed air in the heat exchanger of the gas turbine cycle, about 35-40% of the heat required to raise the compressed air at the inlet to a suitable gas turbine operating temperature, in the range of about 1600.degree. to 2600.degree. F., is provided. This arrangement considerably reduces the fuel requirement for heating the compressor discharge air to the desired gas turbine inlet temperature.
Although many hardware limitations are overcome by using an indirect-fired gas turbine cycle bottomed with a fuel cell cycle as compared to a fuel cell cycle bottomed with a steam turbine cycle, there remain inefficiencies in these types of combined cycles in that additional fuel must be supplied to the system for thermal powering and the requirement of an anode-to-cathode recycle system to produce CO.sub.2 required for the molten carbonate fuel cell cathode reaction. Thus, there is a need for a fuel cell integrated power generation system with improved thermal integration and higher operating efficiencies.